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So I purchased a historic house built in 1920. The house has two HVAC units, one for the upper floor, one for the lower. Total sq ft 3300.

I had been setting the upstairs thermostat at about 83 degrees during the day lowering to 76 after sunset as it is not often used during daytime hours. I was told by my HVAC technician that this was a bad idea due to everything absorbing the heat.. carpeting walls furniture insulation etc.

I have to agree as the unit ran nonstop when attempting to get to a comfortable 76 degree sleeping temperature at night. The house has bedrooms on both floors and the downstairs is generally kept at around 78 during the day reduced for sleeping to 74.

I live in a hot and humid climate where overnight lows can be mid 80s with upwards of 80 percent humidity. Day time highs easily reach 98 to 100.

The downstairs area has a high ceiling . Can someone recommend a good temperature setting for both thermostats that will 1. Not have the AC running non stop 2. Not break the bank to maintain a comfortable temperature. I’ve read many conflicting articles on the temperature spread between the two floors and have come away more confused than ever.

  • What size are the two HVAC systems? Are you open to putting work into this house to get the control layers and systems into good shape? – ThreePhaseEel Jul 21 at 19:59
  • The house has been completely remodeled with Sheetrock and insulation internally. It is a wood frame house and historic regulations prevent any changes to the facade of the house. The systems are sized for the area and were installed during the remodel. I may also need to add that thermal windows are a no and the house has an abundance of windows. The system is maintained seasonally twice per year. – Margo B Jul 21 at 20:40
  • What is the current cladding material? (Brick veneer? Wood clapboards? Something else?) Also, when you say "sized for the area", are you talking about a full ACCA Manual J, Eighth Edition calculation with no "fudge factors" involved and corresponding Manual S sizing, or some sort of "rule of thumb"? – ThreePhaseEel Jul 21 at 21:05
  • Also, will they let you replace cladding/facade materials "like for like"? – ThreePhaseEel Jul 21 at 21:06
  • And what exposures do most of the windows face? – ThreePhaseEel Jul 21 at 21:09
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You ask a tough question and I assume those are the articles you've read that left you with no definitive answer.

Personally, I've found it least expensive and of course most comfortable to set-it-and-forget-it in my last 2 homes...the only ones that had central air. By just maintaining the thermostat's comfort range of a few degrees the system will only activate briefly once or twice an hour typically.

Mine's set to 70 and only on the 95-plus days does it occasionally fall behind at mid-afternoon for an hour. It depends on cloud assistance here and there or wind being present to cool off the roof and siding.

In your case of 2 systems and 2 floors your only real course of action for any further experimentation or "construction" would be to install a door...likely just temporarily with even plastic sheeting taped liberally at the top and bottom.

A door or the enclosure of the top of the stairs (so cold air doesn't fall from upstairs) would allow you to see if turning the upstairs off or up so high during the day is beneficial at all. I'd suggest 74 or 75 all of the time, unless you're actually comfortable at the 78 for the majority of 24-hours.

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If you are uncomfortable upstairs at night during the time it takes the upstairs to go from 83 to 76, then you could turn the thermostat down one, two, or three hours earlier. This will cost more but there is no other way. If you would leave the thermostat at 76 all day, then it would cost you a lot more than that.

The upstairs unit is somewhat undersized if it cannot lower the temperature from 83 to 76 in an hour or two, but that is the unit you have. Check the filter, change it more often and see if that helps.

  • Luckily, the master is downstairs. The upstairs bedroom is for my son. Also, it is not open to the downstairs as it has a hidden staircase that is closed during the day. It is a 3 ton unit for 1000 sq feet. That compared to the high humidity levels and overnight lows. The area in question is directly over the kitchen and sunroom which I am sure exacerbates the problem. Thanks for the insight all. – Margo B Jul 21 at 23:15
  • 3 tons (36,000 BTU/h) is huge for 1000 sq ft. I wonder if the unit is really performing at that level. I would not think that the heat input from the kitchen and sun room below would be enough to account for the inability to cool. What is the thickness of the insulation from the attic? How many windows? – Jim Stewart Jul 21 at 23:20
  • There are 6 barn glass windows on the upper level which I understand can hold heat in and of themselves. The attic may also be an issue as it is vented (large screened vents) and has a weird layout around the room for access above. The wall insulation is approximately 8 inches or so but I think I need to take a look at the roof insulation. The area upstairs is 1000 sq ft but it also incorporates the downstairs laundry area and storage on the same duct work. – Margo B Jul 21 at 23:28
  • @MargoB Where are the air-handlers located in all this? Are the ducts run using rigid metal ducting, or flexible duct? – ThreePhaseEel Jul 21 at 23:53
  • @MargoB -- also, what product was used to insulate the house? Foam (spray or board, open-cell or closed-cell)? Fiberglass (either batt or blown)? Rockwool? Celluose? And which way do those big windows on the upper floor face? – ThreePhaseEel Jul 21 at 23:55

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